70mm Focus Vingnetting?

[Ryan Gallagher]

Quick question... what would lead to experiencing focal plane vignetting

We have two identical lenses for our 70mm setup. One of which does not have a reference for how deep it mounts. In reproducing what we thought we had going last year we are noticing an inability to keep the center in focus relative to the edges. But only on one projector, other one is perfect.

I did check that studio guides are not pinching and warping the film in the gate. For 70mm change we typically find the center of the focus knob travel, and then rough focus with the mount, then fine with the knob.

Lens pair are Kiptar 6.88in/175mm. Thanks for investigation path suggestions.

[Steve Guttag]

First, Kiptar 160mm+ are not rated for 70mm. So, it is by luck that you got one to work. ISCO topped out at 150mm for their normal 70mm format lenses.

Have you tried transposing lenses to see if the problem moves with the lens or stays with the projector?

With the newer flimsy mylar prints that are thinner than the ones of yesteryear, it is incredibly easy for them to buckle. Try removing the studio guides rather than trusting that they are not touching. Verify that the lateral guide roller is not pinching the film too much.

Don't overlook the light and heat at the aperture too. If the light is focused to cause excessive puckering at the gate, that can affect center to edge focus too. See if you can find a 6.75 or 7" lens with a 4" diameter barrel (Kiptakon, B&L Super Cinefor and even the Kollmorgan) had them

[Ryan Gallagher]

I was worried the answer would be something along those lines. lol. Noted. For timing reasons alternate lenses is out of the question this season. I inherited many such issues in this booth it seems.

Swapping them across projectors is a bit unwieldy because one projector takes collets, the other having a turret does not. But I can certainly throw the other one in a spare collet and see if the problem stays or goes away on this non-turret unit.

Will consider backing off or removing studio guides too in a troubleshooting attempt. I'm aware heat can affect it... we have very even matched fields at the moment (after installing the spreader lens and retuning the lamp Z/focus axis of course), so not suspecting that right away.

[Ryan Gallagher]

Don't overlook the light and heat at the aperture too. If the light is focused to cause excessive puckering at the gate, that can affect center to edge focus too. See if you can find a 6.75 or 7" lens with a 4" diameter barrel (Kiptakon, B&L Super Cinefor and even the Kollmorgan) had them

Along those lines, assuming matched intensity and lamp focus... would the "enclosed gate" design of the non-turret model Century JJ be more prone to heat affected pucker issues than the relatively "open" gate design of the turret model? That is one nuance. The focus issue is on our enclosed gate unit. And yes this is a 2024 print (NxNW). Most of my past experiences are with more vintage prints, although we did show The Searchers modern print last season with no issues.

We ran two reels in test today, screening tomorrow.

In hindsight from home, when i checked guide clearance on the bench it was with a loop of black we had handy, I need to recheck in place with the actual print as one is supposed to. (Or just remove them in test) Could be the non-shrunken nature is causing studio guide clearance issues.

[Ryan Gallagher]

It probably points to some other slight gate/trap alignment issue (or our trap not being original to this projector as Tony references in other threads)...

But happy to report that for whatever reason, with this particular print, removing the century JJ studio guides solved the focus issue (definitely film curl of some kind by guide contact). I tried them in the extreme off position first, but no dice. This somewhat explains why this booth ran without them for eons, but we had to resort to them last season for a very weave heavy mag print.

Before that I tried the other lens, problem remained so not in the lens at least..

Ready for a film. Thanks.

[Ryan Gallagher]

Complete. The new NxNW print is a looker too, been out a year now and the heads/tails are starting to show it, but otherwise very nice.

Our programmer keeps claiming we've outdone all prior screenings at the paramount. He's like a broken record at this point. But new 70mm house reels did their part this time!

Perfection is probably like an desert oasis mirage on the horizon, but what else is there to do but keep walking that direction on step at a time.

[Mark Gulbrandsen]

Will consider backing off or removing studio guides too in a troubleshooting attempt. I'm aware heat can affect it... we have very even matched fields at the moment (after installing the spreader lens and retuning the lamp Z/focus axis of course), so not suspecting that right away.

I never left studio guides on any JJ, DP-70's don't even have them! First of all, seems as though people can never clean along the studio guides and behind the bands properly. And others could not properly align the guides, resulting in a groove being worn into the inside of outer guide itself. Not having them is better, and the film is stiff enough laterally that it really doesn't float.

[Ryan Gallagher]

I never left studio guides on any JJ, DP-70's don't even have them! First of all, seems as though people can never clean along the studio guides and behind the bands properly. And others could not properly align the guides, resulting in a groove being worn into the inside of outer guide itself. Not having them is better, and the film is stiff enough laterally that it really doesn't float.

Noted Mark. We lacked them before, until we had a print so weavy we weren't gonna be able to show it. A lot has changed since then, arm spindles trued, new 70mm house reels... so unsure if the print was to blame (but it was a mag acetate print). We'll probably remove the pair on the other projector before the next 70mm and see how it goes.

But as Brad pointed out to me in an email chain, if guides in their fully relaxed position are still causing problems, there is some other alignment issue at play. He was thinking perhaps gate azimuth etc.

[Mark Gulbrandsen]

I somewhat agree with Brad. There are other things to check... But the guides are simply not needed. Can't think of a Euro made projector that has em. My real reason for removing them was the operators were not getting the area under the bands by the aperature plate clean. I saw no difference with or without them when running 70mm target film. And the aperature plate is also securely held in place.